This invention relates to the reaction of carbonaceous material to produce liquid and gaseous hydrocarbons. More particularly, the invention relates to a two stage process for the hydropyrolysis of carbonaceous material to produce gaseous and liquid hydrocarbons including benzene, toluene and xylene.
It is generally well known that carbonaceous material such as coal is converted to liquid and gaseous hydrocarbons when reacted with hydrogen in the presence of high temperature. Common reactors include the fluidized bed reactor which utilizes a vertical upward flow of reactant gases at a sufficient velocity to overcome the gravitational forces on the carbonaceous particles thereby causing movement of the particles in a gaseous suspension. The fluidized bed reactor is characterized by larger volumes of particles accompanied by longer high temperature exposure times to obtain conversion into liquid and gaseous hydrocarbons.
Another common reactor is the entrained flow reactor which utilizes a high velocity stream of reactant gases to impinge upon and carry the carbonaceous particles through the reactor vessel. Entrained flow reactors are characterized by smaller volumes of particles and shorter exposure times to the high temperature gases.
The two stage entrained flow reactor utilizes a first stage to react carbonaceous char with a gaseous stream of oxygen and steam to produce hydrogen, oxides of carbon, and water. These products continue into the second stage where additional carbonaceous material is fed into the stream. The feed reacts with the first stage stream to produce liquid and gaseous hydrocarbons including large amounts of methane gas and char. The movement of the gases between the first and second stages may be by gravity as in a downflow reactor, or by an inertial propelling force as in an upflow reactor.
Prior two-stage gasification processes have been limited primarily to the production of methane and synthesis gas which then must be upgraded for further use. The second-stage products of prior processes include methane carbon monoxide, carbon dioxide, water, hydrogen and ash. The further upgrading that is required is a shift reaction (CO+H.sub.2 O=CO.sub.2 +H.sub.2) which generates hydrogen; this additional hydrogen is necessary for the methanation reaction (CO+3H.sub.2 =CH.sub.4 +H.sub.2 O). In prior processes, a substantial amount of the methane was made in the further upgrading processes.
In addition, the relatively longer exposure times of the carbonaceous material to the high temperatures caused the rapid devolatilization of the particles into various aromatic compounds, followed by the degradation of the aromatics into smaller straight chain or aliphatic compounds and coke. It is believed, however, that by limiting the high temperature exposure time of the carbonaceous particles, the breakdown of the aromatics into smaller aliphatic compounds and coke is inhibited. The resulting product contains significant amounts of benzene, toluene, and xylene, and other light (low boiling) and heavy (high boiling) aromatics and aliphatic hydrocarbon mixtures including methane gas. The advantage of obtaining liquid hydrocarbons including benzene, toluene and xylene is their use as chemical and petroleum feedstocks. The high industry demand for these feedstocks coupled with their low availability, render them a valuable end product.
Prior art two stage processes for the gasification of coal to produce primarily gaseous hydrocarbons include U.S. Pat. Nos. 4,278,445 by Stickler, 4,278,446 by Von Rosenberg, Jr. and 3,844,733 by Donath. Donath describes a process for two-stage downflow gasification of coal to produce methane rich gas. In the first stage, partially reacted recycle char is reacted with steam and oxygen to produce a first stage synthesis gas including hydrogen and oxides of carbon. In the second stage, the synthesis gas is reacted with coal and steam to produce a methane rich gas (after further upgrading) and partially gasified coal char. The Von Rosenberg and Stickler patents utilize a two stage gasification process distinguished by the high velocity flow of gases through the second stage combined with rapid changes in the acceleration of the gases and entrained coal. The high speed accelerations and decelerations increase the rate of mixing and heat transfer with the carbonaceous feed resulting in the high yields of methane gas.
U.S. Pat. No. 3,960,700 by Rosen describes a process for exposing coal to high heat for short periods of time to maximize the production of desirable liquid hydrocarbons and to reduce the gaseous and polymerized products. The Rosen patent describes a process whereby hydrogen obtained from a separate commercial source is mixed with coal inside a heated reactor. The advantages of utilizing the integrated oxygasification and hydropyrolysis process over the Rosen Process are the reduced complexity and improved thermal efficiency. Process complexity is reduced as follows:
(a) Gas cleanup and treatment operates on only one stream, compared to two or three different streams when the oxygasifier and hydropyrolysis reactor operate apart from each other;
(b) The need to recycle and generate and purify hydrogen is eliminated by feeding raw synthesis gas to the hydropyrolysis reactor;
(c) Oxygen consumption in the hydropyrolysis reactor is reduced or eliminated;
(d) Hydrogen (reaction gas) preheater upstream of hydropyrolysis reactor is eliminated;
(e) Cryogenic gas separation is eliminated and replaced by shift and methanation to pipeline gas.
Thermal efficiency in the integrated process is improved as follows:
(a) Heat available at the oxygasifier outlet is used to heat the hydropyrolysis reactants;
(b) Use of indirect heat transfer is reduced in favor of more efficient direct transport of heat;
(c) Compression and fuel costs associated with hydrogen purification and recycle are eliminated.
Clearly, therefore, there is a need for a hydropyrolysis process utilizing both a relatively inexpensive source of hydrogen and heat, and a short second stage high temperature exposure time to produce an end product containing significant amounts of liquid and gaseous hydrocarbons.